Methods and systems for implementing transcendent page caching

ABSTRACT

A method of implementing a shared cache between a plurality of virtual machines may include maintaining the plurality of virtual machines on one or more physical machines. Each of the plurality of virtual machines may include a private cache. The method may also include determining portions of the private caches that are idle and maintaining a shared cache that comprises the portions of the private caches that are idle. The method may additionally include storing data associated with the plurality of virtual machines in the shared cache and load balancing use of the shared cache between the plurality of virtual machines.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.12/716,441, entitled “METHODS AND SYSTEMS FOR IMPLEMENTING TRANSCENDENTPAGE CACHING”, filed on Mar. 3, 2010, which is a continuation-in-part ofU.S. patent application Ser. No. 12/356,389, entitled “METHODS ANDSYSTEMS FOR IMPLEMENTING TRANSCENDENT PAGE CACHING”, filed on Jan. 20,2009. Each of the above-referenced patent applications is incorporatedby reference herein.

COPYRIGHT STATEMENT

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

FIELD OF THE INVENTION

The present invention relates, in general, to physical memory managementin computer systems.

BACKGROUND

Presently, in computer systems, a large portion of physical memory isoften used to store disk data that might possibly be used in the future;if it is not used, that memory is wasted or “idle”. Similar methods areused in a virtualized computer system and, as a result, many virtualmachines (VMs) may each have large amounts of idle memory. One methodcalled “ballooning” allows idle memory from one VM (VM1) to betransferred to another VM (VM2), but ballooning has significantweaknesses, notably that it cannot respond rapidly enough to accommodateconstantly changing demands in the computing environment, and it isdifficult to determine the amount of memory that is needed by VM2 andthat the memory can be safely surrendered by VM1. These deficienciesresult in significantly reduced performance which limits the broadapplication of ballooning. Lastly, in a virtualized system large amountsof physical memory are often left unassigned to any specific VM—or left“fallow”—for short or long periods of time. Because of the large amountsof idle memory in VMs, the limitations of ballooning, and the largeamount of memory left fallow in a virtualized system, physical memory inmost virtualized computer systems in used inefficiently, incurringsignificant performance costs, capital costs (such as the purchase ofunnecessary additional memory), and variable IT costs (i.e., floorspace,power, cooling, etc.). Hence, there is a need for improved methods andsystems in the art of managing physical memory in virtualized computingenvironments.

BRIEF SUMMARY

In some embodiments, a method of implementing a shared cache between aplurality of virtual machines may be presented. The method may includemaintaining the plurality of virtual machines on one or more physicalmachines. Each of the plurality of virtual machines may include aprivate cache. The method may also include determining portions of theprivate caches that are idle and maintaining a shared cache thatcomprises the portions of the private caches that are idle. The methodmay additionally include storing data associated with the plurality ofvirtual machines in the shared cache and load balancing use of theshared cache between the plurality of virtual machines.

In some embodiments, a non-transitory machine-readable medium may bepresented. The medium may include instructions which, when executed byone or more processors, cause the one or more processors to performoperations including maintaining the plurality of virtual machines onone or more physical machines. Each of the plurality of virtual machinesmay include a private cache. The operations may also include determiningportions of the private caches that are idle and maintaining a sharedcache that comprises the portions of the private caches that are idle.The operations may additionally include storing data associated with theplurality of virtual machines in the shared cache and load balancing useof the shared cache between the plurality of virtual machines.

In some embodiments, a system may be presented. The system may includeone or more processors and one or more memory devices. The one or morememory devices may include instructions which, when executed by the oneor more processors, cause the one or more processors to performoperations including maintaining the plurality of virtual machines onone or more physical machines. Each of the plurality of virtual machinesmay include a private cache. The operations may also include determiningportions of the private caches that are idle and maintaining a sharedcache that comprises the portions of the private caches that are idle.The operations may additionally include storing data associated with theplurality of virtual machines in the shared cache and load balancing useof the shared cache between the plurality of virtual machines.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings wherein like reference numerals are usedthroughout the several drawings to refer to similar components. In someinstances, a sub-label is associated with a reference numeral to denoteone of multiple similar components. When reference is made to areference numeral without specification to an existing sub-label, it isintended to refer to all such multiple similar components.

FIG. 1 is a generalized schematic diagram illustrating a computersystem, in accordance with various embodiments of the invention.

FIG. 2 is a block diagram illustrating a networked system of computers,which can be used in accordance with various embodiments of theinvention.

FIGS. 3A and 3B are flow diagrams illustrating transcendent pagecaching, according to embodiments of the present invention.

FIG. 3C is a flow diagram illustrating operations of a hypervisor,according to embodiments of the present invention.

FIG. 4 is a flow diagram illustrating transcendent page caching,according to a further embodiment of the present invention.

FIG. 5 is a block diagram illustrating a system for implementingtranscendent page caching, according to embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

While various aspects of embodiments of the invention have beensummarized above, the following detailed description illustratesexemplary embodiments in further detail to enable one of skill in theart to practice the invention. In the following description, for thepurposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art that the presentinvention may be practiced without some of these specific details. Inother instances, well-known structures and devices are shown in blockdiagram form. Several embodiments of the invention are described belowand, while various features are ascribed to different embodiments, itshould be appreciated that the features described with respect to oneembodiment may be incorporated with another embodiment as well. By thesame token, however, no single feature or features of any describedembodiment should be considered essential to the invention, as otherembodiments of the invention may omit such features.

FIG. 1 provides a schematic illustration of one embodiment of a computersystem 100 that can perform the methods of the invention, as describedherein, and/or can function, for example, as any part of physicalmachine 505 in FIG. 5. It should be noted that FIG. 1 is meant only toprovide a generalized illustration of various components, any or all ofwhich may be utilized as appropriate. FIG. 1, therefore, broadlyillustrates how individual system elements may be implemented in arelatively separated or relatively more integrated manner.

The computer system 100 is shown comprising hardware elements that canbe electrically coupled via a bus 105 (or may otherwise be incommunication, as appropriate). The hardware elements can include one ormore processors 110, including without limitation one or moregeneral-purpose processors and/or one or more special-purpose processors(such as digital signal processing chips, graphics acceleration chips,and/or the like); one or more input devices 115, which can includewithout limitation a mouse, a keyboard and/or the like; and one or moreoutput devices 120, which can include without limitation a displaydevice, a printer and/or the like.

The computer system 100 may further include (and/or be in communicationwith) one or more storage devices 125, which can comprise, withoutlimitation, local and/or network-accessible storage and/or can include,without limitation, a disk drive, a drive array, an optical storagedevice, solid-state storage device such as a random access memory(“RAM”) and/or a read-only memory (“ROM”), which can be programmable,flash-updateable and/or the like. The computer system 100 might alsoinclude a communications subsystem 130, which can include withoutlimitation a modem, a network card (wireless or wired), an infra-redcommunication device, a wireless communication device and/or chipset(such as a Bluetooth™ device, an 802.11 device, a WiFi device, a WiMaxdevice, cellular communication facilities, etc.), and/or the like. Thecommunications subsystem 130 may permit data to be exchanged with anetwork (such as the network described below, to name one example),and/or any other devices described herein. In many embodiments, thecomputer system 100 will further comprise a working memory 135, whichcan include a RAM or ROM device, as described above.

The computer system 100 also can comprise software elements, shown asbeing currently located within the working memory 135, including anoperating system 140 and/or other code, such as one or more applicationprograms 145, which may comprise computer programs of the invention,and/or may be designed to implement methods of the invention and/orconfigure systems of the invention, as described herein. Merely by wayof example, one or more procedures described with respect to themethod(s) discussed above might be implemented as code and/orinstructions executable by a computer (and/or a processor within acomputer). A set of these instructions and/or code might be stored on acomputer-readable storage medium, such as the storage device(s) 125described above. In some cases, the storage medium might be incorporatedwithin a computer system, such as the system 100. In other embodiments,the storage medium might be separate from a computer system (i.e., aremovable medium, such as a compact disc, etc.), and or provided in aninstallation package, such that the storage medium can be used toprogram a general purpose computer with the instructions/code storedthereon. These instructions might take the form of executable code,which is executable by the computer system 100 and/or might take theform of source and/or installable code, which, upon compilation and/orinstallation on the computer system 100 (e.g., using any of a variety ofgenerally available compilers, installation programs,compression/decompression utilities, etc.) then takes the form ofexecutable code.

It will be apparent to those skilled in the art that substantialvariations may be made in accordance with specific requirements. Forexample, customized hardware might also be used, and/or particularelements might be implemented in hardware, software (including portablesoftware, such as applets, etc.), or both. Further, connection to othercomputing devices such as network input/output devices may be employed.

In one aspect, the invention employs a computer system (such as thecomputer system 100) to perform methods of the invention. According to aset of embodiments, some or all of the procedures of such methods areperformed by the computer system 100 in response to processor 110executing one or more sequences of one or more instructions (which mightbe incorporated into the operating system 140 and/or other code, such asan application program 145) contained in the working memory 135. Suchinstructions may be read into the working memory 135 from anothermachine-readable medium, such as one or more of the storage device(s)125. Merely by way of example, execution of the sequences ofinstructions contained in the working memory 135 might cause theprocessor(s) 110 to perform one or more procedures of the methodsdescribed herein.

The terms “machine-readable medium” and “computer readable medium”, asused herein, refer to any medium that participates in providing datathat causes a machine to operate in a specific fashion. In an embodimentimplemented using the computer system 100, various machine-readablemedia might be involved in providing instructions/code to processor(s)110 for execution and/or might be used to store and/or carry suchinstructions/code (e.g., as signals). In many implementations, acomputer-readable medium is a physical and/or tangible storage medium.Such a medium may take many forms, including but not limited to,non-volatile media, volatile media, and transmission media. Non-volatilemedia includes, for example, optical or magnetic disks, such as thestorage device(s) 125. Volatile media includes, without limitationdynamic memory, such as the working memory 135. Transmission mediaincludes coaxial cables, copper wire and fiber optics, including thewires that comprise the bus 105, as well as the various components ofthe communication subsystem 130 (and/or the media by which thecommunications subsystem 130 provides communication with other devices).Hence, transmission media can also take the form of waves (includingwithout limitation radio, acoustic and/or light waves, such as thosegenerated during radio-wave and infra-red data communications).

Common forms of physical and/or tangible computer-readable mediainclude, for example, a floppy disk, a flexible disk, hard disk,magnetic tape, or any other magnetic medium, a CD-ROM, any other opticalmedium, punchcards, papertape, any other physical medium with patternsof holes, a RAM, a PROM, an EPROM, a FLASH-EPROM, any other memory chipor cartridge, a carrier wave as described hereinafter, or any othermedium from which a computer can read instructions and/or code.

Various forms of machine-readable media may be involved in carrying oneor more sequences of one or more instructions to the processor(s) 110for execution. Merely by way of example, the instructions may initiallybe carried on a magnetic disk and/or optical disc of a remote computer.A remote computer might load the instructions into its dynamic memoryand send the instructions as signals over a transmission medium to bereceived and/or executed by the computer system 100. These signals,which might be in the form of electromagnetic signals, acoustic signals,optical signals and/or the like, are all examples of carrier waves onwhich instructions can be encoded, in accordance with variousembodiments of the invention.

The communications subsystem 130 (and/or components thereof) generallywill receive the signals, and the bus 105 then might carry the signals(and/or the data, instructions, etc., carried by the signals) to theworking memory 135, from which the processor(s) 105 retrieves andexecutes the instructions. The instructions received by the workingmemory 135 may optionally be stored on a storage device 125 eitherbefore or after execution by the processor(s) 110.

A set of embodiments comprises systems for implementing physical machine505 in FIG. 5. In one embodiment, user computers 205 and/or servers 215may be implemented as computer system 100 in FIG. 1. Merely by way ofexample, FIG. 2 illustrates a schematic diagram of a system 200 that canbe used in accordance with one set of embodiments. The system 200 caninclude one or more user computers 205. The user computers 205 can begeneral purpose private computers (including, merely by way of example,private computers and/or laptop computers running any appropriate flavorof Microsoft Corp.'s Windows™ and/or Apple Corp.'s Macintosh™ operatingsystems) and/or workstation computers running any of a variety ofcommercially-available UNIX™ or UNIX-like operating systems. These usercomputers 205 can also have any of a variety of applications, includingone or more applications configured to perform methods of the invention,as well as one or more office applications, database client and/orserver applications, and web browser applications. Alternatively, theuser computers 205 can be any other electronic device, such as athin-client computer, Internet-enabled mobile telephone, and/or privatedigital assistant (PDA), capable of communicating via a network (e.g.,the network 210 described below) and/or displaying and navigating webpages or other types of electronic documents. Although the exemplarysystem 200 is shown with three user computers 205, any number of usercomputers can be supported.

Certain embodiments of the invention operate in a networked environment,which can include a network 210. The network 210 can be any type ofnetwork familiar to those skilled in the art that can support datacommunications using any of a variety of commercially-availableprotocols, including without limitation TCP/IP, SNA, IPX, AppleTalk, andthe like. Merely by way of example, the network 210 can be a local areanetwork (“LAN”), including without limitation an Ethernet network, aToken-Ring network and/or the like; a wide-area network (WAN); a virtualnetwork, including without limitation a virtual private network (“VPN”);the Internet; an intranet; an extranet; a public switched telephonenetwork (“PSTN”); an infra-red network; a wireless network, includingwithout limitation a network operating under any of the IEEE 802.11suite of protocols, the Bluetooth™ protocol known in the art, and/or anyother wireless protocol; and/or any combination of these and/or othernetworks.

Embodiments of the invention can include one or more server computers215. Each of the server computers 215 may be configured with anoperating system, including without limitation any of those discussedabove, as well as any commercially (or freely) available serveroperating systems. Each of the servers 215 may also be running one ormore applications, which can be configured to provide services to one ormore clients 205 and/or other servers 215.

Merely by way of example, one of the servers 215 may be a web server,which can be used, merely by way of example, to process requests for webpages or other electronic documents from user computers 205. The webserver can also run a variety of server applications, including HTTPservers, FTP servers, CGI servers, database servers, Java™ servers, andthe like. In some embodiments of the invention, the web server may beconfigured to serve web pages that can be operated within a web browseron one or more of the user computers 205 to perform methods of theinvention.

The server computers 215, in some embodiments, might include one or moreapplication servers, which can include one or more applicationsaccessible by a client running on one or more of the client computers205 and/or other servers 215. Merely by way of example, the server(s)215 can be one or more general purpose computers capable of executingprograms or scripts in response to the user computers 205 and/or otherservers 215, including without limitation web applications (which might,in some cases, be configured to perform methods of the invention).Merely by way of example, a web application can be implemented as one ormore scripts or programs written in any suitable programming language,such as Java™, C, C#™ or C++, and/or any scripting language, such asPerl, Python, or TCL, as well as combinations of anyprogramming/scripting languages. The application server(s) can alsoinclude database servers, including without limitation thosecommercially available from Oracle™, Microsoft™, Sybase™, IBM™ and thelike, which can process requests from clients (including, depending onthe configurator, database clients, API clients, web browsers, etc.)running on a user computer 205 and/or another server 215. In someembodiments, an application server can create web pages dynamically fordisplaying the information in accordance with embodiments of theinvention. Data provided by an application server may be formatted asweb pages (comprising HTML, Javascript, etc., for example) and/or may beforwarded to a user computer 205 via a web server (as described above,for example). Similarly, a web server might receive web page requestsand/or input data from a user computer 205 and/or forward the web pagerequests and/or input data to an application server. In some cases a webserver may be integrated with an application server.

In accordance with further embodiments, one or more servers 215 canfunction as a file server and/or can include one or more of the files(e.g., application code, data files, etc.) necessary to implementmethods of the invention incorporated by an application running on auser computer 205 and/or another server 215. Alternatively, as thoseskilled in the art will appreciate, a file server can include allnecessary files, allowing such an application to be invoked remotely bya user computer 205 and/or server 215. It should be noted that thefunctions described with respect to various servers herein (e.g.,application server, database server, web server, file server, etc.) canbe performed by a single server and/or a plurality of specializedservers, depending on implementation-specific needs and parameters.

In certain embodiments, the system can include one or more databases220. The location of the database(s) 220 is discretionary: merely by wayof example, a database 220 a might reside on a storage medium local to(and/or resident in) a server 215 a (and/or a user computer 205).Alternatively, a database 220 b can be remote from any or all of thecomputers 205, 215, so long as the database can be in communication(e.g., via the network 210) with one or more of these. In a particularset of embodiments, a database 220 can reside in a storage-area network(“SAN”) familiar to those skilled in the art. (Likewise, any necessaryfiles for performing the functions attributed to the computers 205, 215can be stored locally on the respective computer and/or remotely, asappropriate.) In one set of embodiments, the database 220 can be arelational database, such as an Oracle™ database, that is adapted tostore, update, and retrieve data in response to SQL-formatted commands.The database might be controlled and/or maintained by a database server,as described above, for example.

FIG. 3A illustrates a method 300 of performing transcendent pagecaching, according to embodiments of the present invention. At processblock 305, multiple virtual machines (VMs) are established on a physicalmachine. In one embodiment, the physical machine may be system 100 fromFIG. 1, user computer 205 a, 205 b, or 205 c (FIG. 2), or server 215 aor 215 b (FIG. 2). Accordingly, the physical machine may be anycomputing device capable of hosting a number of VMs. Furthermore, theVMs may be established in such a way that each VM utilizes a portion ofthe physical machines resources (i.e., memory, disk, CPU, etc.) tooperate the VM. As such, the physical machine may have additionalresources remaining after allocation of resources to the VMs iscompleted. In one embodiment, those additional resources may be used asa shared cache.

In one embodiment, a shared cache consists of a space in the physicalmachine's memory that may be accessed and/or utilized by any of the VMsindirectly through a programming interface or via a method other thanthe normal direct memory access instructions supported by the processor.For example, a VM may be utilizing all of their resources (i.e., usingto capacity the VM's private cache) and may be able to access the sharedcache in order to supplement their limited resources. By accessing cacheitems (e.g., memory pages) as opposed to accessing the items on a diskstorage, performance can be significantly increased. Hence, the morecache a VM has, the better the performance of the VM. Accordingly, basedon various techniques and algorithms in the art, a VM caches memorypages which will be of greatest importance (i.e., memory pages whichwill most likely be accessed again in the future) in its private cache(process block 310), in order to avoid having to retrieve the memorypages from disk.

Further, because each VM's private cache has a finite size, eventuallythe cache will be filled, and in order to make room for additional (moreimportant) memory pages, some memory pages are removed from the cache.As such, at decision block 315, a determination is made whether anymemory pages in the private caches have been designated to be removed.If no pages in any of the VMs' private caches have been designated to beremoved, then memory pages are continued to be stored in the privatecaches until one or more are designated to be removed.

As such, once a memory page has been designated to be removed from oneor more of the VMs' private caches, the removal is delayed by a virtualmachine shared cache manager (or hypervisor) (process block 320). In oneembodiment, the hypervisor may be a module which has access to theshared cache as well as each of the VMs' private caches. Furthermore,the hypervisor may be configured to manage the memory and resourceallocations from each of the VMs.

Stepping away to FIG. 3C, which illustrates operations of a hypervisorin accordance with aspects of the present invention. In one embodiment,a hypervisor is system software that manages physical resources of aphysical machine. For example, at process block 380, the hypervisordivides and assigns physical resources to one or more virtual machinesas specified by a system administrator. At process block 385, thehypervisor than creates an illusion for the operating environmentrunning in each virtual machine that each is running on a separatephysical machine. At process block 390, the hypervisor isolates thevirtual machines from each other to ensure no one virtual machine canaccidentally or maliciously interfere with another virtual machine.

The delay of removing the memory page triggers a call to the hypervisor(process block 325), and a request is made that the hypervisor allow thememory page designated for removal from the VM's private cache to bestored in the shared cache (process block 330). The decision whether toallow the memory page to be stored in the shared cache may take intoconsideration a number of factors. For example, a usage history for eachVM may be maintained by the hypervisor which tracks how often each VMrequests and/or stores memory pages in the shared cache. Suchinformation can assist the hypervisor in regulating usage of the sharecache by any one VM, and ensure that proper load balancing among the VMsis maintained.

Additional factors may include the usefulness of the memory page to bestored, or the usefulness of the page in comparison to other pages beingrequested to be stored in the shared cache, or parameters specified by asystem administrator. The memory space available in the shared cache mayalso be a factor. Ultimately, the hypervisor is configured to act as agatekeeper to the shared cache to ensure that each VM running on thephysical machine has balanced access to the shared cache.

Accordingly, at decision block 335, it is determined whether the requestto store the memory page in the shared cache is granted. If the requestis denied, then at process block 345, the page is dropped from therequesting VM's private cache, and new memory pages are continued to bestored in the VM's private cache to replace the dropped page (seeprocess block 310). However, if the request is granted, then at processblock 340, the memory page is stored in the shared cache by thehypervisor.

Furthermore, as the VMs continue to perform their designated operations(e.g., launching applications, storing files, deleting files, executingservices, etc.), the VMs will need to access cached memory pages.Referring now to FIG. 3B, which continues method 300 from point A. Atprocess block 350, the VM which stored the memory page in the sharedcache generates a request for the stored memory page. Accordingly, inorder to access the memory page, the VM requests the page from thehypervisor (process block 355). However, because page storage requeststo the shared cache are received by the hypervisor from multiple VMs,not all memory pages stored in the shared cache are able to remain thereindefinitely. After a given amount of time, pages from the shared cachemay be removed in order to make room from other memory pages. Suchdecisions are made by the hypervisor in order to afford all VMs balancedaccess to shared cache.

Hence, the shared cache is checked by the hypervisor to determine if therequested memory page is still stored within the shared cache (processblock 360). At decision block 365, it is determined whether the page isstored within the shared cache. If it is determined that the memory pageis not stored within the shared cache, then at process block 380, the VMis indicated as such, and the VM accesses the VM's disk storage toretrieve the requested memory page. However, if the page is stored inthe shared cache, then at process block 370, the hypervisor retrievesthe page from the shared cache. Further, at process block 375, theretrieved page is transferred to the requesting VM for usage by the VM,thus, saving the VM from needing to retrieve the page from the muchslower disk storage.

FIG. 4 illustrates a method 400 of implementing transcendent pagecaching according to embodiments of the present invention. At processblock 405, multiple VMs may be similarly established on a physicalmachine. Furthermore, a portion of the physical memory of the physicalmachine is partitioned for use by the multiple VMs (process block 410).Accordingly, each VM is allocated a certain amount of memory from thephysical machine for use in the VM's private cache; however, at anygiven time at least a portion of the allocated memory may be idle. Forexample, one or more of the VMs may be performing few or no functionssuch that some or all of their allocated memory is idle (i.e.,available).

Oftentimes, while some VMs sit idle, other VMs are working at maximumcapacity utilizing all of their allocated recourses; however, in normalcircumstances, the active VMs cannot access the unused resources fromthe idle VMs, and as such the resources are wasted. Nonetheless, in ashared (or transcendent) cache, according to aspects of the presentinvention, through the use of the hypervisor, the idle memory isreclaimed and used in the transcendent cache (process block 415).

At process block 420, the reclaimed idle memory is collected into a“pool” of shared memory which can then be accessed by the VMs running onthe physical machine. Accordingly, the size of the transcendent cachevaries as the amount of idle memory available changes. Furthermore,access to the transcendent cache is judiciously controlled by thehypervisor (process block 425). Similar to aspects described in method300, the hypervisor grants or denies requests to use the transcendentcache, and equally allows VMs to use the shared cache. As such, theotherwise wasted memory is efficiently used by other VMs which are inneed of additional resources.

In an alternative embodiment, this transcendent cache allocation schememay be implemented in any environment where shared resources arepresent. For example, in a server pool (or farm) which pool resources,the idle (or unused) resources may be captured and access to them may becontrolled/allocated by a hypervisor.

Turning now to FIG. 5, a system 500 is illustrated for implementingtranscendent page caching according to embodiments of the presentinvention. In one embodiment, system 500 may include a physical machine505. In one embodiment, physical machine 505 may include a hypervisor510, virtual machines (VMs) 1-N and a transcendent page cache 515. Eachof VMs 1-N are allocated a portion of physical machine 505's memory (notshown) to be used as the VMs' private caches. However, as discussedabove, the VMs 1-N do not use all of the allocated memory at all time,some of the memory is idle. Therefore, the idle memory is used byhypervisor 510 to create transcendent cache 515.

As such, as VMs 1-N are in need of additional cache space to storememory pages, VMs 1-N can request that hypervisor 510 allow them tostore those pages in transcendent cache 515. Such a store may beexecuted using a put( )command or similar such command. Furthermore,when any of VMs 1-N need to access a stored page(s) in transcendentcache 515, the VM can execute a get( )command, or similar such command.Such a command will request that hypervisor 510 access the requestedpage and transfer it to the requesting VM.

In a further embodiment, after a VM has successfully stored a memorypage in transcendent cache 515, the VM may subsequently delete thecontent of the stored memory page (e.g., delete the file, the addressreference, etc.), and as such, the page will be stale (or outdated) andwill need to be removed. Accordingly, when the content of the page isdeleted by the VM, the VM sends a request for hypervisor 510 to executea flush( )or similar command on the memory page. Hence, the stale pagewill not inadvertently be accessed and cause a data consistency erroramong other similar types of errors.

While the invention has been described with respect to exemplaryembodiments, one skilled in the art will recognize that numerousmodifications are possible. For example, the methods and processesdescribed herein may be implemented using hardware components, softwarecomponents, and/or any combination thereof. Further, while variousmethods and processes described herein may be described with respect toparticular structural and/or functional components for ease ofdescription, methods of the invention are not limited to any particularstructural and/or functional architecture but instead can be implementedon any suitable hardware, firmware, and/or software configurator.Similarly, while various functionalities are ascribed to certain systemcomponents, unless the context dictates otherwise, this functionalitycan be distributed among various other system components in accordancewith different embodiments of the invention.

Moreover, while the procedures comprised in the methods and processesdescribed herein are described in a particular order for ease ofdescription, unless the context dictates otherwise, various proceduresmay be reordered, added, and/or omitted in accordance with variousembodiments of the invention. Moreover, the procedures described withrespect to one method or process may be incorporated within otherdescribed methods or processes; likewise, system components describedaccording to a particular structural architecture and/or with respect toone system may be organized in alternative structural architecturesand/or incorporated within other described systems. Hence, while variousembodiments are described with—or without—certain features for ease ofdescription and to illustrate exemplary features, the various componentsand/or features described herein with respect to a particular embodimentcan be substituted, added and/or subtracted from among other describedembodiments, unless the context dictates otherwise. Consequently,although the invention has been described with respect to exemplaryembodiments, it will be appreciated that the invention is intended tocover all modifications and equivalents within the scope of thefollowing claims.

What is claimed is:
 1. A method of implementing a shared cache between aplurality of virtual machines, the method comprising: maintaining, by acomputer system, the plurality of virtual machines on one or morephysical machines, wherein each of the plurality of virtual machinesincludes a private cache; determining, by the computer system, portionsof the private caches that are idle; maintaining, by the computersystem, a shared cache that comprises the portions of the private cachesthat are idle; storing, by the computer system, data associated with theplurality of virtual machines in the shared cache; and load balancing,by the computer system, use of the shared cache between the plurality ofvirtual machines.
 2. The method of claim 1, wherein each of theplurality of virtual machines are isolated from each other such that noone of the plurality of virtual machines interferes with the privatecache of another one of the plurality of virtual machines.
 3. The methodof claim 1, further comprising: storing, by the computer system, amemory page in a first private cache associated with a first virtualmachine in the plurality of virtual machines; determining, by thecomputer system, that the memory page has been designated for removalfrom the first private cache; determining, by the computer system, thatthe memory page can be stored in the shared cache based on theload-balancing of the use of the shared cache between the plurality ofvirtual machines; and storing, by the computer system, the memory pagein the shared cache.
 4. The method of claim 3, further comprising:receiving, by the computer system, a request from a second virtualmachine in the plurality of virtual machines for the memory page; andtransferring, by the computer system, the memory page from the sharedmemory to a second private cache associated with the second virtualmachine.
 5. The method of claim 1, wherein the shared cache comprises amemory location that is not part of any of the private caches of any ofthe plurality of virtual machines.
 6. The method of claim 1, wherein theshared cache is managed by a hypervisor that manages physical resourcesof the one or more physical machines and divides and assigns thephysical resources to the plurality of virtual machines.
 7. The methodof claim 1, wherein the shared cache is dynamically resized based on achange in the size of the portions of the private caches that are idle.8. The method of claim 1, further comprising limiting, by the computersystem, the amount of each of the portions of the private caches thatare idle that can be used as part of the shared cache.
 9. Anon-transitory machine-readable medium comprising instructions which,when executed by one or more processors, cause the one or moreprocessors to perform operations comprising: maintaining the pluralityof virtual machines on one or more physical machines, wherein each ofthe plurality of virtual machines includes a private cache; determiningportions of the private caches that are idle; maintaining a shared cachethat comprises the portions of the private caches that are idle; storingdata associated with the plurality of virtual machines in the sharedcache; and load balancing use of the shared cache between the pluralityof virtual machines.
 10. The non-transitory machine-readable medium ofclaim 9, wherein each of the plurality of virtual machines are isolatedfrom each other such that no one of the plurality of virtual machinesinterferes with the private cache of another one of the plurality ofvirtual machines.
 11. The non-transitory machine-readable medium ofclaim 9, wherein the operations further comprise: storing a memory pagein a first private cache associated with a first virtual machine in theplurality of virtual machines; determining that the memory page has beendesignated for removal from the first private cache; determining thatthe memory page can be stored in the shared cache based on theload-balancing of the use of the shared cache between the plurality ofvirtual machines; and storing the memory page in the shared cache. 12.The non-transitory machine-readable medium of claim 11, wherein theoperations further comprise: receiving a request from a second virtualmachine in the plurality of virtual machines for the memory page; andtransferring the memory page from the shared memory to a second privatecache associated with the second virtual machine.
 13. The non-transitorymachine-readable medium of claim 9, wherein the shared cache comprises amemory location that is not part of any of the private caches of any ofthe plurality of virtual machines.
 14. The non-transitorymachine-readable medium of claim 9, wherein the shared cache is managedby a hypervisor that manages physical resources of the one or morephysical machines and divides and assigns the physical resources to theplurality of virtual machines.
 15. The non-transitory machine-readablemedium of claim 9, wherein the shared cache is dynamically resized basedon a change in the size of the portions of the private caches that areidle.
 16. The non-transitory machine-readable medium of claim 9 whereinthe operations further comprise limiting the amount of each of theportions of the private caches that are idle that can be used as part ofthe shared cache.
 17. A system comprising: one or more processors; andone or more memory devices, wherein the one or more memory devicescomprise instructions which, when executed by the one or moreprocessors, cause the one or more processors to perform operationscomprising: maintaining the plurality of virtual machines on one or morephysical machines, wherein each of the plurality of virtual machinesincludes a private cache; determining portions of the private cachesthat are idle; maintaining a shared cache that comprises the portions ofthe private caches that are idle; storing data associated with theplurality of virtual machines in the shared cache; and load balancinguse of the shared cache between the plurality of virtual machines. 18.The system of claim 17, wherein the one or more memory devices furthercomprises additional instructions that cause the one or more processorsto perform additional operations comprising: storing a memory page in afirst private cache associated with a first virtual machine in theplurality of virtual machines; determining that the memory page has beendesignated for removal from the first private cache; determining thatthe memory page can be stored in the shared cache based on theload-balancing of the use of the shared cache between the plurality ofvirtual machines; storing the memory page in the shared cache; receivinga request from a second virtual machine in the plurality of virtualmachines for the memory page; and transferring the memory page from theshared memory to a second private cache associated with the secondvirtual machine.
 19. The system of claim 17, wherein the shared cachecomprises a memory location that is not part of any of the privatecaches of any of the plurality of virtual machines.
 20. The system ofclaim 17, wherein the shared cache is dynamically resized based on achange in the size of the portions of the private caches that are idle.